Various systems have been developed that provide warnings and advisory indications of hazardous flight conditions. Among such systems are systems generally known as ground proximity warning systems (GPWS) which monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that inadvertent contact with terrain is imminent. Among the flight conditions normally monitored by such systems are radio altitude and rate, barometric altitude and rate, air speed, flap and gear positions. These parameters are monitored and an advisory signal and/or warning signal is generated when the relationship between the parameters is such that terrain impact is likely to occur. Typical examples of such systems are disclosed in U.S. Pat. Nos. 3,715,718; 3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221; 3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and 4,319,218, all assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference.
While the above-referenced systems do provide advisory and warning signals in the event of proximity to terrain, the warnings generated by such systems are based solely upon flight conditions of the aircraft and do not provide any navigational information nor a visual indication of the terrain below the aircraft. Consequently, the sensitivity of such systems must be adjusted to provide adequate warnings when a hazardous flight condition exists without generating false or spurious warnings. However, such an adjustment can result in a compromise that may still result in nuisance warnings over terrain unique to particular geographic areas and shorter than desired warning times in yet other geographic areas.
Several attempts have been made to improve upon such ground proximity warning systems utilizing ground-based navigational information. For example, U.S. Pat. Nos. 4,567,483; 4,646,244; 4,675,823; and 4,914,436 all disclose ground proximity warning systems which monitor the position of the aircraft relative to stored terrain data in order to provide modified ground proximity warnings. However, the utility of such systems is limited. For example, the systems disclosed in U.S. Pat. Nos. 4,567,483 and 4,914,436 disclose ground proximity warning systems which utilize navigational data to modify predetermined warning envelopes surrounding certain particular airports. U.S. Pat. Nos. 4,646,244 and 4,675,823 disclose terrain advisory systems which utilize various ground-based navigational inputs and stored terrain data to provide various ground proximity warning systems based on the position of the aircraft. However, as with the systems described above, none of these systems provide the pilot with navigational information or an indication of the terrain below the aircraft.
Recently an enhanced ground proximity warning system (EGPWS) which includes a terrain advisory and warning system has been developed. In this system data stored in a terrain data base is used in conjunction with aircraft position information to generate a visual terrain advisories and warning on a cockpit display, such as weather radar. An object of the EGPWS is to provide the crew with a terrain awareness system which provides increased warning times to the pilot of an aircraft of a hazardous flight condition while minimizing nuisance warnings. An example of a EGPWS is described in U.S. patent application Ser. No. 08/509,642 filed on Jul. 31, 1995 and which is assigned to the assignee of this application. However, the terrain display feature of the EGPWS provides at best only a simple contoured display of terrain relative to the altitude of the aircraft. Further, when the aircraft is more than 2000 feet above the highest terrain within the range of the display, no terrain features are shown.
However, there are some circumstances in which it would be desirable to provide the pilots with a contoured display of terrain below the aircraft when the aircraft is not in an immediate hazard condition. For example, it would be useful to be able to provide the flight crew with a means of assessing the potential terrain conflicts which might occur if a rapid descent is initiated from the aircraft's normal flight path. A loss of cabin pressure, for instance, might require the crew to initiate an emergency decent and it would be very useful if the crew had a map of the terrain below the aircraft. This would be particularly important when the aircraft is traversing mountainous terrain. Since the terrain display of the EGPWS normally does not display terrain more than 2000 feet below the aircraft this system is not normally available for planning a rapid decent from a relatively high altitude.